(1) Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and in particular to technology for reducing noise generated during standby.
(2) Description of the Related Art
Halogen lamp fixing devices fix toner images to recording sheets by passing recording sheets through a fixing nip formed where a pressing roller presses against a fixing roller that has a halogen lamp provided therein as a source of heat. The fixing roller may, for example, be a metal bar that encloses a halogen lamp and whose outer circumferential surface is covered by an elastic layer made of silicone rubber or the like. The pressing roller is also a metal bar whose outer circumferential surface is covered by an elastic layer made of silicone rubber or the like.
During standby, i.e. while the image forming apparatus is waiting to print, the fixing roller and the pressing roller continue to press against each other without rotating. If heat from the fixing roller is transferred to the pressing roller in this state via the fixing nip, only the portion of the pressing roller at the fixing nip will heat up, whereas other portions will gradually dissipate heat and cool down. This is because the silicone rubber forming the elastic layer of the pressing roller has low thermal conductance, making it difficult for heat to be transferred to portions of the pressing roller other than at the fixing nip. Such a situation leads to temperature variation in the circumferential direction along the outer surface of the pressing roller.
Therefore, when printing after returning from a standby state, unevenness in fixing may occur due to temperature variation along the pressing roller. One approach to resolve this problem is to cause the fixing roller and the pressing roller to idly rotate during standby. Doing so allows the heat from the fixing roller to be transferred evenly across the outer circumferential surface of the pressing roller, thus preventing temperature variation, which in turn prevents uneven fixing.
Technology has also been proposed to place a halogen lamp inside the pressing roller as well and to heat the pressing roller while causing the rollers to rotate idly. In this case, in order to prevent hot offset caused by excessive heat from the fixing roller or the like, the length of time of idle rotation is adjusted based on a predicted value of accumulated heat of the fixing roller and the like (see Japanese Patent Application Publication No. 20044-26191).
A problem occurs, however, in that if the fixing roller and other rollers rotate idly when the image forming apparatus is not printing, the user may find the resulting operation noise to be unpleasant. This problem is particularly salient in settings such as an office in which people do desk work for long hours, as it is difficult to avoid hearing the noise if the image forming apparatus is located nearby.